Colours hide in more places than fill=""
An SVG can set colour in fill and stroke attributes, in style="" attributes, in <style> blocks, and on gradient stop-color. SVGShift finds hex, rgb() and named colours in all of these, normalises them, and replaces every occurrence of the one you change.
currentColor for icon systems
Setting fills to currentColor makes an inline SVG inherit the CSS color of its parent, so one icon can be purple in a button and white in a dark header without separate files.